This invention relates to a substrate which is mountable with an optical component such as optical semiconductor device and optical device including optical fiber and lens, a method for producing such a substrate, and an optical module using such a substrate.
In recent years, there has been the demand for larger capacity and more functions of optical communication systems and, accordingly, miniaturization, higher integration and lower production costs have been required for optical devices such as optical transmitters and optical receivers. Particularly, in order to reduce the assembling cost of optical devices, attention has been given to the technique for mounting optical components such as optical semiconductor devices, optical fibers and lenses on a single substrate, in particular, to the so-called passive alignment technique such as an optical hybrid mounting technique, and an alignment-free mounting technique using a silicone platform.
According to the above techniques, an optical axis adjustment and an optical coupling can be established without an aligning operation only by mounting optical components on the substrate with respect to a positioning groove formed in the substrate or mounting optical components in mounting grooves formed in the substrate. This enables a significant reduction in the assembling costs. In order to mount optical components without aligning operation, the optical component mounting grooves formed in the substrate are required to have highly precise dimensions of top opening width and depth. Further, the relative positional relationship between the grooves is required to be precise in the order of sub-micrometers or less than one millionth of a meter.
However, there has been the big problem that a plurality of grooves whose respective depths are different from each other cannot be simultaneously made by a single etching operation for the following reasons.
Generally, in the case where a groove is made in a substrate made of a silicon monocrystal by an etching, there will occur an undercut 100 below the non-patterned photoresist layer 200 as shown in FIG. 8. More specifically, in the case that a V-shaped groove is formed in a substrate made of a silicon monocrystal having a (100) surface as a principle surface by anisotropic etching using an alkaline solution (etching solution: KOH (43 vol %) solution, solution temperature 60xc2x0 C.), an undercut occurs in proportion to the depth of etching as can be seen from a graph of FIG. 9.
The undercut amount represented by the vertical axis in FIG. 9 means a difference between a designed pattern width and an actual top opening width (V-shaped groove width) of a V-shaped groove after etching. It will be seen from FIG. 8 that the undercut amount (2L) is calculated by subtracting a designed width (DW) of the patterned hole from an actual width (AW) of the V-shaped groove, that is, 2L=AWxe2x88x92DW. To make a groove having a depth of 800 xcexcm, for example, it is necessary to apply etching for approximately 50 hours, and an undercut of about 30 xcexcm occurs during this etching. It means that in the case of making a groove having a final depth of 30 xcexcm, the designed pattern width should be at 0 xcexcm. This is to say that a groove having a width of 30 xcexcm or smaller cannot be practically made. The occurrence of undercuts will make it impossible to form a plurality of grooves having different depths in a single substrate by a single etching.
Further, in the case of making a plurality of grooves having trapezoidal cross sections and different depths to their bottom surfaces, it has been difficult to simultaneously form them by a single etching.
In view of this problem, a substrate for mounting an optical component (hereinafter, referred to as xe2x80x9coptical component mounting substratexe2x80x9d) which is formed with different depth grooves has been conventionally produced by steps shown in FIGS. 10A to 10E. In order to facilitate the description, in these figures, forming of two V-shaped grooves in a substrate will be described.
First, as shown in FIG. 10A, a protection film 52 made of a silicon oxide or a silicon nitride is formed on the entire top surface of a substrate 51 made of a silicon monocrystal. The protection film 52 is resistant to an etching solution for etching the substrate 51. Further, a photoresist film 53 is formed on the entire top surface of the protection film 52. Patterning is applied to the photoresist film 53 by photolithography using a photomask for part of a V-shaped groove to be formed in a first etching. Thereafter, the part of the protection film 52 which corresponds to the V-shaped groove is also removed by a silicon oxide etching solution.
Subsequently, as shown in FIG. 10B, a V-shaped groove 54 is formed by etching the surface part where the substrate 51 is exposed using an etching solution containing sodium hydroxide (NaOH), potassium hydroxide (KOH), tetramethylammonium hydroxide (TMAH), etc.
Subsequently, as shown in FIG. 10C, a protection film 55 made of a silicon oxide or a silicon nitride is formed on the entire top surface of the substrate 51 including the V-shaped groove 54 after the protection film 52 is entirely removed, and a photoresist film 56 is formed on the entire top surface of the protection film 55. A photomask for a V-shaped groove to be formed in a second etching is positioned with reference to a marker provided on the substrate 51 to position the photomask, and patterning is applied to the photoresist film 56 by photolithography. Thereafter, the part of the protection film 55 which corresponds to the groove to be formed by the second etching is removed.
Subsequently, as shown in FIG. 10D, a groove 57 is formed by the same procedure as the V-shaped groove 54, and the protection film 55 is finally removed to produce an optical component mounting substrate J1 formed with the grooves as shown in FIG. 10E.
In the above-mentioned conventional method, it will be apparent that high-precision patterning cannot be accomplished for the second formed photoresist film 56 and protection film 55 because of the presence of the V-shaped groove 54 formed in the first etching. In view of this problem, Japanese Unexamined Patent Publication No. 3-132031 proposes making of a plurality of V-shaped grooves having different depths by applying patterning only to the top flat surface of the substrate. However, this proposal cannot successfully eliminate the likelihood that the position of a second-placed photomask is different from that of a first-placed photomask, which causes undesired displacement of V-shaped grooves having different depths.
Specifically, a reference marker is provided at a specified position of the substrate, e.g., at an end of the substrate. In the first and second etchings, the photomask is arranged over the substrate by positioning a marker provided on the photomask with respect to the reference marker on the substrate. The photoresist film or protection film is likely to have an irregular thickness around the previously-formed groove, which makes it difficult to distinguish the marker provided on the substrate. Further, an exposure device, e.g., a contact-type exposure device, is usually incapable of positioning in the order of sub-microns.
Even if the above problems could be cleared, the substrate is liable to be warped due to a heat history since the positioning marker provided on the substrate have been subjected to various heat treatment processes. This warping causes shift of the positioning marker.
It will be seen to be extremely difficult to make the marker provided on the photomask agree with the marker provided on the substrate. Accordingly, in the above-mentioned conventional method, the greater the number of grooves having different depths becomes, the greater the number of placing photomask becomes, which consequently accumulates displacements of markers, and finally results in the unacceptable disagreement among grooves.
It is an object of the present invention to provide a substrate for mounting an optical component, a method for producing an optical component mounting substrate, and an optical module which are free from the problems residing in the prior art.
According to the invention, a substrate for mounting an optical component is formed with a first groove and a second groove at least. The second groove has a depth greater than the first groove. The first groove and the second groove have a relationship with each other represented by the following equation,
(2D sin xcex8)/Rxe2x89xa7C
where D denotes a depth of the first groove, xcex8 denotes an angle between a horizontal plane and a slanted surface of the first groove (0xc2x0 less than xcex8 less than 90xc2x0), R=F/E (E denotes an etching rate of a slanted surface of the first groove, F denotes an etching rate of a bottom surface of the groove, C denotes a top opening width of the groove.
The first groove and the second grooves are formed by forming a protection film on an entire top surface of the substrate, forming pattern holes corresponding to the first and second grooves in the protection film by a single mask having holes corresponding to the first and second grooves, forming a protection seal on a pattern hole corresponding to one of the first and second grooves, applying anisotropic etching to the substrate through the not-sealed pattern hole to thereby form the other of the first and second grooves, removing the protection seal, forming another protection seal on the groove formed by anisotropic etching, applying anisotropic etching to the substrate through the seal-removed pattern hole to thereby form the remaining one of the first and second grooves, and removing the protection film and the another protection seal from the substrate.
The inventive substrate is mounted with an optical component to produce an inventive optical module.
Thus, an optical component mounting substrate formed with a plurality of grooves having different sizes and depths can be formed to have highly precisely dimensions. Further, optical devices can be accurately mounted on the substrate, thereby enabling production of an optical module having an excellent performance.